Boston Celtics head coach Doc Rivers shouts from the bench during the first half of an NBA basketball game against the Miami Heat at the TD Garden in Boston, Sunday, Jan. 27, 2013. The Celtics defeated the Heat 100-98. (AP Photo/Steven Senne)— AP

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Boston Celtics head coach Doc Rivers shouts from the bench during the first half of an NBA basketball game against the Miami Heat at the TD Garden in Boston, Sunday, Jan. 27, 2013. The Celtics defeated the Heat 100-98. (AP Photo/Steven Senne)
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Miami Heat head coach Erik Spoelstra, left, and forward LeBron James (6), right, look on from the bench in the fourth quarter of an NBA basketball game against the Boston Celtics at the TD Garden in Boston, Sunday, Jan. 27, 2013. The Celtics won 100-98. (AP Photo/Steven Senne)— AP

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Miami Heat head coach Erik Spoelstra, left, and forward LeBron James (6), right, look on from the bench in the fourth quarter of an NBA basketball game against the Boston Celtics at the TD Garden in Boston, Sunday, Jan. 27, 2013. The Celtics won 100-98. (AP Photo/Steven Senne)
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Miami Heat guard Ray Allen acknowledges people in the crowd during warmups before their NBA basketball game against the Boston Celtics at TD Garden in Boston, Sunday, Jan. 27, 2013. The Celtics won 100-98. (AP Photo/Steven Senne)— AP

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Miami Heat guard Ray Allen acknowledges people in the crowd during warmups before their NBA basketball game against the Boston Celtics at TD Garden in Boston, Sunday, Jan. 27, 2013. The Celtics won 100-98. (AP Photo/Steven Senne)
/ AP

BOSTON 
The Celtics' grip on the last playoff spot in the Eastern Conference already was slipping.

Now they'll have to hang on without Rajon Rondo.

Boston's star point guard will miss the rest of the season and undergo surgery for a torn anterior cruciate ligament in his right knee. The Celtics won without him on Sunday, beating the Miami Heat 100-98 in double overtime.

But what will the Celtics (21-23) do without him for the remaining 38 games with just a 2 1/2-game lead over the ninth-place Philadelphia 76ers?

Rondo had triple-doubles in his last two games, giving him five for the season. He brought the ball up, got it to the right people and even improved his weak shooting so much that his 48.4 field goal percentage was third best among NBA guards.

He was averaging career highs of 13.7 points and 5.6 rebounds along with 11.1 assists this season. And he was chosen as the starting point guard for the Eastern Conference in the All-Star game in Houston on Feb. 17.

Now he's done.

The club said he was hurt late in Friday night's 123-111 loss, also in double overtime, in Atlanta.

"We still like our chances in the Eastern Conference," said Paul Pierce, who had his own triple-double Sunday with 17 points, 13 rebounds and 10 assists. "Responsibilities definitely go up when you have Rondo out of the game. (I'm) more of a facilitator. (I) have to (do) a little bit of everything for this ball club."

Kevin Garnett led the Celtics with 24 points and 11 rebounds and they responded well without Rondo. Their other guards - Avery Bradley, Courtney Lee, Jason Terry and Leandro Barbosa - each played at least 25 minutes and combined for 35 points.

"New guys are going to get an opportunity now," Pierce said.

The Celtic's prospects were bleak even before Rivers learned about 25 minutes before the game that Rondo wouldn't play. They had lost six straight games, their longest slide in six seasons. The Heat had won their last four and had the best record in the East.

At first, Rondo thought he had a hamstring injury, Rivers said. Then he was ruled out of the game with what the team said was a hyperextended right knee. An MRI was done and Rivers learned about the ACL tear during the game.

"He's known to play through injuries," Pierce said. "If Rondo can suit up, he's going to suit up. So he didn't suit up today and we knew it wasn't good."

Rivers told his players of the severity of Rondo's injury after the game.

"Everyone was really happy for the win," Pierce said. "It brought a dark cloud in this room when you heard the news."

Even LeBron James, who led all scorers with 34 points, expressed sympathy.

"As much as I've been a rival with Boston over the years, I never want to see anyone go down," James said. "It's terrible."

James had a chance to put the Heat ahead after Pierce's 22-footer gave Boston a 99-98 lead with 31 seconds left in the second overtime. But James missed a 12-foot jumper with 6.8 seconds to go. Pierce got the rebound and was fouled.

He sank the first shot. Then, as a fan shouted "This one's for Rondo," he missed the second.

Miami had one last chance, but Shane Battier missed a long jumper at the buzzer.

"They defended that very well," Heat coach Erik Spoelstra said. "There are about three different options to it, four different options to it. They defended each one of them."

The Heat also could have won in the first overtime, but Dwyane Wade, who had 17 points, missed a jumper as the buzzer sounded.

Boston could have avoided the first overtime when Pierce inbounded from behind his backboard to Terry with 2 seconds left. But Terry's shot from the top of the key was short. The Heat had tied it on a 3-pointer by James with 7 seconds remaining in regulation after Ray Allen missed a 3-pointer with 15 seconds to go.

This game was Allen's first in Boston since he left the Celtics after five seasons and signed as a free agent with Miami. He scored 21 points.

The crowd gave him a standing ovation when highlights of his career were shown on the video board above center court during a timeout with 5:33 left in the first quarter. At the Miami bench, he raised his left hand in recognition.

"When I saw it, just all those emotions came streaming back from all the great things we did here," Allen said. "I'll always be a Celtic in my mind."

Allen entered the game about a minute after the tribute and was booed when he touched the ball. The boos came down again when he took two free throws - missing the first, making the second - three minutes into the second quarter.

Now the Celtics have neither member of their starting backcourt from the past five seasons, Allen and Rondo.

Rondo's injury "puts this team and the rest of the guys in a position to be ready to step up," Pierce said. Sunday's win "was a perfect example. We showed we are capable."

NOTES: It was Miami's first game in Boston since it won Game 6 of the Eastern Conference finals behind James' 45 points. ... James and Chris Bosh each had 16 rebounds for the Heat. ... The last time the Celtics played consecutive double-overtime games was on March 11 and 13, 1951.